BEIJING -- Foreign journalists were barred from Beijing's Tiananmen Square on Wednesday as police fanned out across the vast plaza on the eve of the 20th anniversary of the bloody crackdown on 1989 pro-democracy protests.

Authorities blocked social networking and image-sharing Web sites such as Twitter and Flickr and confined dissidents to their homes or forced them to leave Beijing, as they ramped up efforts to prevent online discussions about or commemorations of those who died in the military assault on demonstrators on the night of June 3-4 1989.

Dozens of uniformed and plainclothes police guarded entrances to Tiananmen Square in the heart of Beijing that was the epicenter of the student-led protests. Officers demanded identification and turned away those whose passports said they were journalists.

The sweeping measures have been imposed even though there were few signs of efforts to mark the protests within mainland China, where the government squelches all discussion of them.

Beijing has never allowed an independent investigation into the military's crushing of the 1989 protests, in which possibly thousands of students, activists and ordinary citizens were killed. And young Chinese know little about the events, having grown up in a generation that has largely eschewed politics in favor of nationalism and economic development.

But authorities have been steadily tightening surveillance over China's dissident community ahead of this year's anniversary, with some leading writers already under close watch or house arrest for months.

Ding Zilin, a retired professor and advocate for Tiananmen victims whose teenage son was killed in the crackdown, said by telephone that a dozen officers blocked her and her husband from leaving their Beijing apartment Wednesday morning.

Another leading dissident voice, Bao Tong, was taken by police to southeastern China over the anniversary, said his son, Bao Pu.

Bao Tong, 76, is the former secretary to Zhao Ziyang, the Communist Party leader deposed for sympathizing with the pro-democracy protesters.

As in past years, foreign media reports on issues related to the protests in print, on television or the Internet were blocked.

Over recent days, journalists attempting to film on the square or interview dissidents have been detained for several hours on apparently trumped-up charges of creating disturbances, according to the Foreign Correspondents Club of China.

However, the blocking of social networking sites marks a new chapter in the authorities' attempts to muzzle dissent, one that testifies to the burgeoning influence of such technology among young Chinese in an authoritarian society where information is tightly controlled.

Government Internet monitors have shuttered message boards on more than 6,000 Web sites affiliated with colleges and universities, apparently to head off any talk about the 1989 events, according to the Hong Kong-based Information Center for Human Rights and Democracy.

Numerous Chinese sites were also disabled Wednesday, including mini-blogging site Fanfou and video sharing site VeryCD. Notices on their home pages said they would be closed through Saturday for "technical maintenance."

The text-messaging service Twitter and pictures on photo sharing site Flickr could not be accessed within China starting Tuesday. Video sharing site YouTube has been blocked within China since March.

Jason Khoury, spokesman for Yahoo, which owns Flickr, said no explanation had been given and the company believed the restrictions were "inconsistent with the right to freedom of expression."

Officials from Twitter did not respond to a request for comment.

In Hong Kong, where the anniversary of the protests is openly commemorated, a second dissident who took part in the 1989 events was denied entry to the territory on Wednesday.

U.S. Consulate General spokesman Dale Kreisher said the decision to deport Xiang Xiaoji, an American citizen, was "particularly regrettable in light of Hong Kong's well-known reputation as an open society."

Xiang had planned to attend Hong Kong's annual candlelight vigil to honor victims of the crackdown.

Christopher Bodeen

Associated Press Writer

The Associated Press is the backbone of the world's information system serving thousands of daily newspaper, radio, television and online customers with coverage in all media and news in all formats. It is the largest and oldest news organization in the world, serving as a source of news, photos, graphics, audio and video.